1. Field of the Invention
The present invention relates to image processing for the medical images captured by various types of medical image acquisition apparatuses (modalities).
2. Description of the Related Art
A photoacoustic tomograph (PAT) excites an absorbing substance in a subject by irradiating the subject with optical pulses, and detects the photoacoustic signal generated by the thermoelastic expansion of the absorbing substance, thereby imaging properties associated with the light absorption of the subject. In other words, the PAT images the distribution of optical energy accumulation amounts (the distribution of optical energy absorption densities) in a subject with respect to irradiation light. In addition, based on this distribution, the PAT images the distribution of optical absorption coefficients of the subject concerning an irradiation wavelength. In addition, it is possible to image the states of substances constituting the subject (for example, oxygen saturation of hemoglobin) based on the distributions of optical absorption coefficients concerning a plurality of wavelengths.
These images are expected to visualize information associated with new blood vessels generated inside and outside a malignant tumor such as cancer. These images will be collectively referred to as “PAT images (photoacoustic tomographic images)” hereinafter.
A PAT is designed to irradiate a human body with near-infrared pulses of low energy, and hence has difficulty in imaging a deep portion of the human body as compared with X-rays. According to Japanese Patent Laid-Open No. 2010-088627, the PAT designed to measure breasts is used in a form such that a breast is held by two flat plates (to be referred to as “holding plates” hereinafter) and imaged while the thickness of the breast is reduced. For this reason, when performing a diagnosis using both a PAT and another modality such as a magnetic resonance imaging (MRI) apparatus, it is possible to allow a doctor to efficiently perform the diagnosis by performing deformation alignment (deforming one image to align it with the other image) in consideration of compression deformation caused by holding the breast.
A method of aligning a PAT image with an MRI image includes an image matching method. For example, literature 1 described below discloses a technique for alignment between an X-ray mammography (MMG) image obtained by imaging a breast compressed by flat plates and an MRI image of the breast. More specifically, a deformed MRI image is generated by performing a physical deformation simulation by compression with flat plates with respect to an MRI image, a pseudo-MMG image is generated from the deformed MRI image, and alignment is performed by matching between the pseudo-MMG image and the actually captured MMG image.
Literature 1: Angela Lee, et al., “Breast X-ray and MR image fusion using finite element modeling,” Proc. Workshop on Breast Image Analysis in conjunction with MICCAI 2011, pp. 129-136, 2011
In addition, literature 2 described below discloses a technique of evaluating the shape of a breast after deformation which is obtained as a result of performing a physical deformation simulation by compression with flat plates with respect to an MRI image based on the two-dimensional shape of the breast which is extracted from an MMG image.
Literature 2: C. Tanner, et al., “Breast Shapes on Real and Simulated Mammograms,” Proc. Int. Workshop on Digital Mammography 2010 (IWDM 2010), LNCS 6136, pp. 540-547, 2010
Furthermore, an attempt has been made, as diagnosis support by alignment between a plurality of modalities different from the above, to generate (clip) and present an image of a slice (to be referred to as a “corresponding slice” hereinafter) corresponding to an imaging slice of an ultrasonic image from a reference image such as a computed tomographic image (CT image) or MRI image which is three-dimensional image data. For example, Japanese Patent No. 03871747 discloses a technique of aligning a CT image or MRI image as a reference image with a subject in advance, measuring the position and posture of an ultrasonic probe relative to the subject, and aligning the ultrasonic image with the reference image.
However, since the PAT and the MRI differ in their characteristics to be imaged, all the structures depicted in an MRI image do not match those on a PAT image. It is therefore difficult to execute high-accuracy alignment by performing only image matching. For this reason, it is necessary to manually input a plurality of corresponding points between two coordinate systems.